1. Field of the Invention
The present invention relates to conversion of digital data to analog form for application to a display device for forming an image thereon and more particularly, to developing an analog intensity signal from digital data for forming an image on a display.
2. Description of the Prior Art
Digital data used for forming images on video displays e.g., cathode ray tubes (CRT's), typically represent intensity information for each separate point on the scanned raster of the video display. This digital data is coupled through a digital-to-analog (D/A) converter to generate an analog voltage which is applied to the electron gun of the CRT for controlling the intensity of each point in the scanned raster. A typical prior art circuit for accomplishing this is shown in FIG. 1. Included in FIG. 1 are memory planes 12, 14 and 16, a look-up table 18 and a D/A converter 20. In an embodiment for driving a black and white display, each memory plane 12, 14 and 16 corresponds to a separate and distinct intensity level of an image to be displayed and contains a "bit-map" (a pattern of ones and zeros) of an entire image to be displayed for that respective intensity level. Bits corresponding to the same location of the bit-map for each plane, are simultaneously coupled to the addressing inputs of look-up table 18. Table 18 is preloaded via a central processor (not shown) with a distinct multi-bit number representative of a given intensity, for each combination of bits by which it can be addressed. For example, although in the FIG. 1 embodiment only 8 memory locations can be addressed by the three-bit address, if the multi-bit numbers stored in look-up table 18 have 8 bits each, the addressing selects 8 out of a possible 256 different intensity levels. The data loaded into look-up table 18 causes its output to correspond to the intensity level of the brightest (most intense) of the memory planes, for each bit of the bit-map. Thus, output intensity levels for each location of the "composite" or combined bit-map are sequentially provided to D/A converter 20 for conversion to an analog voltage. Since D/A converter 20 must determine an analog voltage for each point of the bit-map, it must operate at a relatively fast rate, e.g., 50 MHz.
It should be noted that in a prior art embodiment having a color display, three circuits of the type shown in FIG. 1 would be used, one for generating each of red, green and blue intensity signals, which signals would be applied to respective ones of red, green and blue drive circuits of a color CRT. Thus, three high-speed D/A converters would be required.
D/A converters which operate at such high speeds are relatively expensive as compared to D/A converters which operate at much slower speeds, e.g., 500 KHz. Additionally, fast D/A converters have greater power consumption as compared to slower D/A converters.
It is desirable to reduce the cost and power consumption of D/A converter circuits used in digital data conversion.